The invention deals, in general, with lifting gear and more particularly with the use of such lifting gear in sensitive environments, such as the reactors of nuclear power stations. More specifically, the invention relates to a method for regulating the operation of a load compensation device for such handling gear, as well as to the load compensator that uses this method.
Although the invention will be more specifically described in conjunction with its application to the nuclear industry, it will be clearly understood that its scope is not limited thereto.
The core of the reactor in a nuclear power station consists, as is known, of a certain number of nuclear fuel assemblies put in place at a transverse bearer or core plate at the bottom of the reactor vessel.
These fuel assemblies can be handled independently of one another given that, after a certain amount of irradiation time, the irradiated fuel assemblies need either to be replaced or to be positioned differently within the core in order to make the radiation of energy in the bottom of the core uniform.
These fuel assemblies are handled using a handling machine, also known as a refuelling machine, capable of moving in a horizontal plane above the pool that covers the reactor core, the machine being equipped with an operating truck also capable of moving in another horizontal direction within the machine.
The truck in fact comprises lifting gear which usually consists of a vertical telescopic mast that can be unfolded, at the end of which there is a gripper capable of engaging with the upper end of the nuclear fuel assemblies. The telescopic mast can be moved in the vertical direction by a lifting means which usually consists of a motorized winch, on the winch drum of which a cable or chain or any equivalent member is wound and which somewhere within the system comprises one or more return pulleys.
Generally speaking, nuclear fuel assemblies consist of rods comprising sintered pellets of actual fuel, and joined together by means of spacer grids distributed along the height of the assembly.
Given that the various nuclear fuel assemblies are positioned side by side in the core plate, and come into contact with one another, especially at the spacer grids, it has been observed that the assemblies become snagged at the grids, especially during operations of lifting or of fitting assemblies with respect to neighbouring assemblies.
In the context of a lifting operation, that is to say the removal or repositioning of a fuel assembly, this snagging results in overloading at the lifting gear, especially overloading of the cable, and this overload needs to be detected immediately so that the refuelling machine winch motor can be stopped.
The reason for this is that, assuming such an overload were not to be detected, or, assuming it were to take too long for the winch motor to be stopped, the grids of the snagged assemblies would be liable to sustain damage and there would be a risk that the cohesion of the assembly itself would suffer.
The same phenomenon occurs when a fuel assembly is being inserted into the core, the only difference here being that the overload this time is underload, which means that the tension in the cable or in the chain is reduced, with the result that the fuel assembly is no longer necessarily positioned vertically.
In order to alleviate this severe drawback, a load compensator intended to be positioned at the truck of the refuelling machine has been proposed, for example in document EP-B-0,292,413 in the name of the Applicant.
Such a device fundamentally comprises:
a stationary frame secured to the truck and comprising two end stops; PA1 a slider intended to slide in the frame between these end stops; PA1 an outer bell housing provided with means capable of cooperating with the slider; PA1 an overload cylinder arranged between the slider and the frame, especially one of the end stops; PA1 an underload cylinder arranged between the slider and the outer bell housing. PA1 a stationary internal bell housing secured to the bed on which the handling gear rests, and inside which an upper piston and a lower piston can move in translation between stops formed within the said bell housing, and between the upper piston and the bottom of the said bell housing, respectively; PA1 a mobile external bell housing that can cooperate with the lower piston and to which is secured one of the ends of the handling member, especially a cable or a chain, from the other end of which the load is hung; PA1 an overload pneumatic cylinder which extends between the upper piston and the stationary internal bell housing, and an underload pneumatic cylinder which extends between the upper piston and the lower piston, the cylinders being connected to a source of pressure or exhausted by a series of electric directional control valves.
The end of the cable or of the chain of the lifting gear is fixed directly or indirectly to the outer bell housing.
Furthermore, an electro-pneumatic circuit for modulating the supply to the cylinders is provided, and acts as a function of the variations in load.
These variations in load are detected by means of a load cell which, as a function of previously determined and set thresholds, brings about the pressurizing of the underload cylinder and/or overload cylinder, respectively, by discrete amounts.
Although these pre-established and therefore fixed pressures do indeed compensate for the positive or negative variations in load, it has, however, also been observed that bearing in mind the lack of control over the effective movement of the cable, the sum of the actions to which the fuel assembly being handled is subjected, namely the movement which caused it to snag and the reverse movement brought about by the compensator, leads to an absolute displacement of the fuel assembly in the opposite direction to the initial movement.
It has now been established that the fuel assemblies that coexist within one and the same core may be of different types and makes and that, in particular, the locations of the spacer grids in each of the assemblies differs and may therefore cause undercontrolled snagging in the opposite direction, precisely because the absolute displacement of the fuel assembly being handled is in the opposite direction from the initial movement.
Furthermore, it has become evident that bearing in mind the speed with which the compensator moves, a relative movement between fuel assemblies is brought about, and the instantaneous speed of this movement is higher than the speed allowed for handling these assemblies.
It has consequently seemed essential to employ precise control over the operation of the compensator, especially to eliminate any reverse movement of the fuel assembly being handled.